A semiconductor laser device of the kind mentioned in the opening paragraph is known, for example, from J.Appl. Phys 50 (5), 3122-3132 (May 1979). In this publication, an aluminum oxide layer is used as coating. The fun of the aluminum oxide layer is inter alia to provide for protection at the area of the emanation of the laser beam against attack of the emanation surfaces under the influence of water vapor present.
Presumably, however, water vapor is not the only cause of problems at the emanation surface and oil present during the different operations may also play a part with an aluminum oxide layer as coating, for example, due to the fact that the oil at the surface of the aluminum oxide layer is decomposed and scatters the emanating laser beam.